The present invention relates to voltage controlled crystal oscillators, and more particularly, to circuitry for preventing the crystal oscillator from jumping from a desired frequency of oscillation to an undesired frequency.
Crystals, and other mechanical resonators, can be made to oscillate at a plurality of frequencies. In particular, a crystal can be cut in many different ways such that its characteristics may be chosen according to the orientation of a cut relative to the crystalline structure. For any particular cut, there is often a fundamental frequency where the crystal is antiresonant and a fundamental frequency where the crystal is series resonant. Typically a crystal oscillator will oscillate somewhere between these two fundamental frequencies where the loop gain of the oscillator is sufficiently high and has the correct phase shift for oscillation. It is also not uncommon for crystal oscillators to be designed to oscillate at a harmonic of the fundamental frequency of the crystal as determined by external circuitry.
A mechanical resonator, even when oscillating at a fundamental frequency, has some mechanical excitation at harmoic frequencies. For various reasons, such as startup instability or transients, it is possible for the mechanical resonator to oscillate at one of the harmonic frequencies above the intended oscillation frequency due to circuit components such as stray capacitances or stray inductances. Thus, the mechanical resonator can be driven to oscillate at one of the harmonic frequencies above the fundamental frequency by the external circuit, or the crystal can be resonated at parasitic harmonic frequencies by components such as stray capacitances or stray inductances.
The frequency of oscillation of a mechanical resonator is determined by external circuitry. The VCXO oscillator of the exemplary embodiment is a portion of a phase locked look (PLL) of an integrated circuit chip in a television receiver, and is a component of the digital processing apparatus of a stereophonic audio signal, as will be discussed more fully hereinafter. If the gain of the oscillator amplifier is high enough at a harmonic frequency where the phase shift of the crystal is favorable, the circuit can jump to the harmonic frequency and will establish a stable oscillation at said harmonic frequency.
As used herein, television receiver is intended to include any television signal processor such as a VCR or a monitor, with or without a display device such as a CRT.
Accordingly it is desirable to provide an economical means for assuring that a mechanical resonator oscillator will oscillate at the intended frequency and not at harmonic frequencies of the intended frequency.